1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display which is characterized by its small thickness, small occupied volume and light weight is used on many electrical products such as mobile phones and personal computers. Since a liquid crystal, which is a component member of the liquid crystal display, does not emit light by itself, the liquid crystal display requires an illuminating apparatus for illuminating the liquid crystal when used in a place where sufficient sunlight or light from a lighting equipment is not available. Accordingly, it is also desired that the illuminating apparatus be compact in size and low in power consumption. However, when the illuminating apparatus gets smaller, the workability in assembling the apparatus and the durability of a die for the apparatus are lowered. In particular, in a spread illuminating apparatus in which a light source is disposed on a side surface of a resin substrate and a light reflection pattern is formed on an upper surface of the resin substrate to uniformly emit light coming from the light source toward the whole screen of the liquid crystal display, the light reflection pattern is fine and therefore the workability and the durability of the die are further lowered. A conventional spread illuminating apparatus having a light reflection pattern formed on an upper surface of a resin substrate to attain a uniform illumination will be described hereinbelow.
For example, Japanese Patent Application Laid-open No. 10-208530 discloses a spread illuminating apparatus using a back-light illuminating system, in which a light reflection pattern (stair-like light reflection pattern) is formed such that fine optical elements having plural rows of prisms triangular in section provided on an upper surface of a resin substrate substantially wedge-like in section are disposed parallel to the optical axis of a bar-like light source disposed on a side surface of the resin substrate. The inclination angle of the prism rows is set so that the emission of the maximum brightness is directed perpendicular to the screen, i.e., the front intensity is maximized, at an area farthest from the light source in the distribution of light emitted from the resin substrate after coming from the light source and being reflected at the prism rows. The inclination angles of the prism rows are set different from each other so that the front intensity of the emission distribution of light is equal to the above-described front intensity at the area farthest from the light source irrespective of the distance from the light source.
In the above-described conventional example, the uniformity in the direction of light emitted from the resin substrate and the amount of light emitted are controlled by the shape of the prism (inclined surfaces by setting angle of inclination), and it has been difficult to optimally control the uniformity of total light by means of a controlling method using these two inclined surfaces as parameters. To solve this problem, Japanese Patent Application Laid-open No. 11-242222 discloses a spread illuminating apparatus using a front light illuminating system, in which a light reflection pattern is formed such that a plurality of grooves triangular in section and a plurality of flat portions adjacent to the grooves are provided on an upper surface of a transparent substrate (resin substrate). For example, the light reflection pattern is formed parallel to the optical axis of a light source disposed on a side surface of the transparent substrate, wherein the depth of the grooves is set to be constant and the interval of adjacent grooves (the width of the flat portion) is set to be in inverse proportion to the distance from the light source so that reflected light is uniformly spread on the whole screen of the illuminating apparatus. Light emitted from the transparent substrate is controlled by a total of three surfaces, specifically the flat portion and two inclined surfaces (two surfaces of the groove of triangular section) forming the light reflection pattern, which gives more appropriate light distribution than the control by the above-described stair-like light reflection pattern.
In the spread illuminating apparatus described above, the grooves (prisms) formed on the upper surface of the transparent substrate are triangular in section and a bottom of a valley part, that is a vertical angle of the groove, is sharp so that a molding die therefor has a sharp projection-like tip. Accordingly, the molding die can not be easily machined, and also its durability is not good enough.
The present invention has been made in the light of the above problems. It is an object of the present invention to provide a spread illuminating apparatus which can easily obtain desired optical characteristics for light emitted from a transparent substrate and at the same time improve the machinability and durability of a molding die for the transparent substrate on which a light reflection pattern is formed.
In order to solve the above problems, according to a first aspect of the present invention, in a spread illuminating apparatus in which a bar-like light source is disposed along and close to a side surface of a transparent substrate made of a light-transmissible material, and a light reflection pattern comprising a plurality of grooves and a plurality of substantially flat portions adjacent to the grooves is formed on an upper surface of the transparent substrate in parallel to the optical axis of the light source, the grooves are each shaped substantially trapezoidal in section with their bottom defining a shorter side. Light coming from the light source is reflected at the grooves and the substantially flat portions both constituting the light reflection pattern and emitted from the transparent substrate. And, the grooves formed trapezoidal can improve the machinability and durability of the molding die.
According to a second aspect of the present invention, in the spread illuminating apparatus of the first aspect, the depth of the grooves gradually increases in proportion to the increase in distance from the light source, whereby an excellent distribution of light emitted from the transparent substrate can be obtained.
According to a third aspect of the present invention, in the spread illuminating apparatus of the first aspect, the width of the grooves is in inverse proportion to the distance from the light source. An excellent distribution of light emitted from the transparent substrate can be obtained by varying the width of the grooves according to the distance from the light source.
According to a fourth aspect of the present invention, in the spread illuminating apparatus of the first aspect, the width of the substantially flat portions is in inverse proportion to the distance from the light source. An excellent distribution of light emitted from the transparent substrate can be obtained by varying the width of the substantially flat portions according to the distance from the light source.
According to a fifth aspect of the present invention, in the spread illuminating apparatus of the first aspect, the grooves are each formed stair-like with a plurality of bottom surfaces stepped.
According to a sixth aspect of the present invention, in the spread illuminating apparatus of any one of the first to fifth aspects, the exterior angle to the angle formed by an inclined surface of the grooves positioned closer to the light source and a bottom surface of the grooves is 35xc2x0 to 55xc2x0, and the inclination angles of both the surface of the substantially flat portions and the surface of the bottom part of the grooves are xe2x88x925xc2x0 to +5xc2x0 with respect to a lower surface of the transparent substrate. An improved light distribution can be obtained by setting the inclination angles of the inclined surface and the bottom surface of the grooves, and the substantially flat portions to respective specified values.